The Clouds of Andromeda

The Andromeda galaxy is an icon of astronomy. But those red, diffuse, glowing clouds are a mystery.

Rogelio Bernal Andreo

The image above is as baffling as it is gorgeous.

First, kudos go to my pal Rogelio Bernal Andreo, who took this magnificent shot. It shows the Andromeda galaxy, the closest big spiral galaxy to our own, and in fact the other big member of our neighborhood galaxy minicluster called the Local Group. At 2.5 million light-years away, it’s bright enough to see with the naked eye from moderately dark sites and shows quite a bit of detail even through small telescopes.

Rogelio’s image is unusual. First of all, it shows a huge area of the sky; Andromeda is several times the apparent size of the full Moon in the sky (see here for a comparison). Hold up your hand, and fold in your pinky and thumb; the width and length of your three middle fingers extended at arm’s length would be about the same area of sky as the photo.

The image is a composite of “natural” color imaging using red, green, and blue filters to mimic what we’d see with our eyes, together with a “luminance” or white-light image, taken with no filter. That adds detail and depth to the image; together astrophotographers call it an LRGB image.

But there’s more. He also took many hours worth of observations using a filter that lets through a very specific wavelength, or color, of light. This red light, called Hα (literally, “H alpha”) is emitted by warm hydrogen gas. Since hydrogen is so common throughout the Universe, an Hα filter is very useful; you can use it to accentuate nebulae and galaxies.

The red clouds in the image are hydrogen nebulae. They’re extremely faint; Rogelio had to work very hard to make them visible above the background light of the sky. In the photo their brightness has been magnified by a substantial amount, so the image isn’t “natural” in that sense; if Andromeda had been scaled on the same brightness range as the clouds, the galaxy would be vastly overexposed, a huge white blob blotting out everything around it.

So the image isn’t really what your eyes would see through a telescope but has been adjusted to show these two very different views simultaneously. Still, it’s beautiful for sure … and very odd.

The big question is, what are those clouds?

On his website (and in a follow-up on his Facebook page), Rogelio describes his observations, their history, and his research into the glowing clouds. He points out he loves chasing faint, diffuse clouds. Called galactic cirrus (or the fancier Integrated Flux Nebulae), these are generally very thin streamers of dust—grains of silicates or long carbon molecules—strewn throughout space inside our galaxy. They’re exceedingly faint, and difficult to image. But if the dust is warm, it can glow in infrared. So Rogelio checked professional observatory infrared images of that area in the sky, but the clouds seen in them don’t fit well to what he observed. That seems to rule out galactic cirrus.

However, he did see them, barely, in a pair of surveys that mapped the sky in Hα. He shows this on his page, and looking at them I’m confident these clouds are real, and consist of glowing hydrogen.

Rogelio also argues, and I agree, that the clouds aren’t physically associated with the Andromeda galaxy. At that distance those clouds would have to be huge, tens of thousands of light-years across, far too large to make any sense. We do see clouds of gas this big in the Universe, but generally speaking they don’t display the structure in the image. For example, that one above the galaxy near the top center shows a long, flat, bright edge. Features like that are common in smaller gas clouds a few light-years in size. You get them when an expanding (or rapidly moving) gas cloud hits material outside it. The gas piles up, forms a sharp edge, and gets denser and brighter. But something like that would be highly unusual, to say the very least, in a cloud of galactic size.

So clearly these are gas clouds inside our own Milky Way, and we’re looking through and past them to the much more distant Andromeda galaxy. But that’s still odd. Nebulae like that are usually heated up and lit by nearby massive stars. The stars emit ultraviolet light, which pumps energy into the gas, and it responds by glowing like a neon sign (literally). But I don’t see any stars close by that could do that.

So why are they glowing?

Given the lack of nearby luminous stars, my first guess was that they are colliding with lower density gas around them. That would explain the one cloud with the sharp edge, but the others are even more diffuse, so I’m not sure that explanation works for them.

I have another idea. Perhaps there’s enough ambient ultraviolet light from massive stars spread out in space that, combined, they can cause these clouds to softly glow. This would be a sort of background UV light, very faint, but enough to trigger the nebulae into emitting Hα light. While that’s a guess, it seems plausible … and I don’t see anything else that makes much sense.

Let me say here that I love this. Andromeda is one of the best studied objects in the entire sky, yet here are objects in the same field of view that have essentially escaped notice all this time. That’s understandable; Andromeda is so bright that faint clouds are ridiculously hard to see, but our technology and techniques are getting so good that the previously hidden is becoming revealed. And on top of that, it also took the dedication of someone like Rogelio to pursue this.

The next step, I should think, is to find a research astronomer who can take an interest in this and get even deeper images and take spectra of these clouds (to determine their chemical composition as well as motion, which can help nail down their distance). There are lots of fascinating questions to answer here. How common are these clouds? How old are they—are they recently cast off by dying stars, or primordial, dating back to when the galaxy was young? Are they everywhere in the sky, or do they tend to be clumped near the galactic plane? And to me, the most interesting question of all: What’s lighting them up?

They’re certainly lighting me up. I hope we can find out more about these elusive beasts soon.